1. Field of the Invention
The present invention relates to an optical transmission system. In particular, the present invention pertains to a multiple wavelength bidirectional lightwave amplifier.
2. Related Art
An optical transmission system transmits information from one place to another by way of a carrier whose frequency is in the visible or near-infrared region of the electromagnetic spectrum. A carrier with such a high frequency is sometimes referred to as an optical signal, an optical carrier, or a lightwave signal.
An optical transmission system includes a plurality of optical fibers. Each optical fiber includes a plurality of channels. A channel is a specified frequency band of an electromagnetic signal, and is sometimes referred to as a wavelength. One link of an optical transmission system typically has a transmitter, the optical fiber, and a receiver. The transmitter converts an electrical signal into the optical signal and launches it into the optical fiber. The optical fiber transports the optical signal to the receiver. The receiver converts the optical signal back into an electrical signal.
An optical transmission system that transmits more than one channel over the same optical fiber is sometimes referred to as a multichannel system. The purpose for using multiple channels in the same optical fiber is to take advantage of the unprecedented capacity offered by optical fibers. Essentially, each channel has its own wavelength, and all wavelengths are separated enough to prevent overlap.
One way to transmit multiple channels is through wavelength division multiplexing, whereupon several wavelengths are transmitted in the same optical fiber. Typically, four channels are interleaved by a multiplexer, launched into the optical fiber, and separated by a demultiplexer at a receiver. Wavelength division demultiplexing elements separate the individual wavelengths using frequency-selective components such as optical gratings or bandpass filters.
Optical signals traveling over long distances need to be regenerated periodically to compensate for fiber loss, sometimes referred to as signal attenuation. Fiber loss reduces the average signal power reaching the receiver. Because optical receivers need a certain amount of power in order to recover the optical signal accurately, the transmission distance of the optical signal is limited by fiber loss.
Optical signal regeneration sometimes utilizes optoelectronic regenerators. A typical optoelectronic regenerator employs a receiver-transmitter pair that detects the incoming optical signal, converts it into an electrical signal, amplifies and refines the electrical signal, and then converts the amplified electrical signal back into a corresponding optical signal. However, optoelectronic regenerators are quite complex and expensive for multichannel systems. Additionally, the electronic components in optoelectronic regenerators cause transmission system bandwidth to be limited. In other words, the difference between the lowest wavelength transmitted and the highest wavelength transmitted is very narrow. Multichannel lightwave systems benefit considerably when optoelectronic regenerators are replaced by much simpler optical amplifiers because the bandwidth of optical amplifiers is much larger than that of optoelectronic regenerators. Loss compensation is carried out in an optical amplifier by amplifying the optical signal directly, without converting it to an electrical signal. In either case, regeneration boosts the signal level and corrects for transmission impairments.
One characteristic of multichannel optical transmission systems concerns the bidirectional application of the optical fiber. In some systems, one set of channels travels in an east-to-west direction in the optical fiber, while another set of channels travel in a west-to-east direction. In order to compensate for signal attenuation, conventional regenerators must utilize two separate gain blocks, one for the incoming east-to-west set of channels and another for the incoming west-to-east set of channels.
Another characteristic of a multichannel optical transmission system is that noise is generated at different points along the length of the system. More particularly, the gain block of the optical amplifier signal regenerator adds noise to the optical signal because of spontaneous emission during amplification. As more noise is added to the optical signal, coupled with the fact that the optical amplifier amplifies the weakest channel the least, the signal-to-noise ratio of the weaker channels becomes degraded. Signal-to-noise ratio is the ratio of the amplitude of the channel to that of the noise. Signal-to-noise ratio is quantified through a parameter called noise figure. Noise figure (F.sub.n) is defined as ##EQU1## where SNR refers to the power contained in the channel.
Still another characteristic of conventional multichannel optical transmission items is that each of the incoming channels may arrive at the signal regenerator input with a different amplitude. This occurs because of fiber attenuation, fiber dispersion, and non-uniform regenerator spacing.
Fiber attenuation is a decrease in average optical power, and results from loss due to scatter, loss due to imperfections in the fiber, and loss due to absorption. Rayleigh scatter occurs when a portion of the lightwave bounces off the uneven areas of the fiber surface, losing energy in the process. Fiber imperfections can stem from the fact that the particular fiber was manufactured using older technology and from the fact that the fiber has bends, either large or small.
Loss due to absorption can be either intrinsic or due to impurities. Intrinsic loss is a basic property of optical fiber whereby because electromagnetic phenomena the energy in the optical signal is absorbed and not recoverable at the receiver. Impurity loss occurs primarily because the presence of water vapors and other impurities cause optical signal energy absorption similar to loss due to intrinsic absorption.
Equally important is the fact that fiber attenuation due to absorption is different at different wavelengths. Two wavelengths that have minimal attenuation are 1300 nm and 1550 nm. The typical attenuation of an optical signal operating 1300 nm, 35 dB/100 km, is much greater than the attenuation of an optical signal operating at 1550 nm, which is 20 dB/100 km. Furthermore, within what is called the 1550 nm transmission window, then the attenuation will be the least at 1550 nm and increase as the operating wavelength moves away from 1550 nm. The same holds true for operating in the 1300 nm transmission window. Unfortunately, it is not feasible to operate at only the wavelength that ensures the least attenuation because that would defeat the objective of having a wide bandwidth of operating wavelengths.
Fiber loss is only one cause for differing input amplitudes. Dispersion in the optical fiber also causes channel amplitudes to vary. The optical fiber is a very thin wire made of glass, or silica, enclosed by cladding. The cladding is designed to reflect the lightwaves into the core of the silica wire. At times, lightwaves reflect back and forth between the core and the cladding so much that the reflected lightwaves are delayed and arrive at the transmission system receiver after the primary lightwave arrives. The delayed lightwaves can be troublesome depending on the type of optical fiber used. Multimode dispersion occurs in what are termed multimode fibers. Single mode fibers eliminate the effects of multimode dispersion, but introduce chromatic dispersion when operating within the 1550 mn transmission window. Step-index fibers have minimal chromatic dispersion operating at 1300 nm, but significant chromatic dispersion at 1550 nm. Dispersion-shifted fibers experience minimum chromatic dispersion operating within the 1550 nm transmission window. As a practical matter, there is no guarantee what type of optical fiber will be utilized for each link of an optical transmission system that spans an entire continent.
Another cause for differing input amplitudes stems from non-uniform regenerator spacing. For example, one channel may originate in Seattle, and another channel in New York. The ultimate destination of each channel is Washington, D.C. In that case, the channel originating in Seattle travels further than the channel originating in New York. Because signal regenerators are placed at convenient as opposed to uniform locations along the route from origin to destination, it is difficult to ensure that the two optical channels arrive at their common destination with the same signal amplitude. a typical gain block of an optical amplifier signal regenerator receives the composite unidirectional signal and amplifies the channel with the greatest incoming amplitude, to the detriment of the weaker channels.
One problem with conventional multichannel optical transmission systems is that the dice in amplification between any two channels must be tolerable. It is desirable to have equal amplitudes for each wavelength. Moreover, the signal-to-noise ratio in any given channel must also be within acceptable limits. This condition is not met when utilizing conventional signal regenerators because of unequal channel amplitude, unequal channel amplification, and the addition of noise in each channel.
Another problem with conventional signal regenerators is that the design is quite complex, expensive, and has poor upgrade capability. For example, in order to operate at higher data rates without changing regenerator spacing, a designer would have to replace most of the system equipment. This is because the electronic elements are normally tuned to the operating parameters of a particular system. If it becomes necessary to increase system capacity, the electronic elements must be replaced. If the gain block section of the signal regenerator is optoelectronic, then the entire regenerator must be replaced. Likewise, it is expensive to have two amplification and refinement stages to service bidirectional optical signals.